Somatostatin is a polypeptide hormone that acts on multiple target organs. It inhibits a number of cell effects including secretion and proliferation. It acts through a seven transmembrane receptor that is coupled to numerous signalling mechanisms which include adenylate cyclase, ion channels and phosphatases. Our aim is to use metabolic labelling to investigate the phosphorylation pattern of various phosphotyrosine proteins stimulated by somatostatin. This is done in a mouse fibroblast cell stably transfected with a somatostatin receptor subtype. We found a 44kD protein that is dephosphorylated in response to somatostatin. Through the use of mass spectrometry we would like to identify this protein and then assess the relationship between protein dephosphorylation and enzyme activity. Additionally, depending on the identification of this protein, we hope to further elucidate the downstream effectors in the signalling pathway of the somatostatin receptor.